Process for the separation of an aqueous mixture of trioxane and formaldehyde and corresponding applications

ABSTRACT

Process for the separation of an aqueous mixture of trioxane and formaldehyde and corresponding applications. The aqueous trioxane and formaldehyde mixture has first trioxane:formaldehyde ratio. The process includes the steps of reacting the aqueous mixture of trioxane and formaldehyde with urea, and separation an exiting vapor phase having a second trioxane:formaldehyde ratio that it higher than the first trioxane:formaldehyde ratio. An aqueous mixture of trioxane and formaldehyde coming from a reactor in which trioxane is being synthesized or from a distillation column in which a prior aqueous mixture of trioxane and formaldehyde can be separated from the excess formaldehyde. Raw material for the production of urea-formaldehyde glues or resins can be formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process allowing mixtures of trioxane andformaldehyde in the presence of water, defining a firsttrioxane:formaldehyde ratio, to be separated.

2. Background Art

A number of steps is required for the preparation of trioxane, whichexplains the large number of patents published.

Generally speaking, trioxane is prepared starting out from formaldehydeby a trimerization reaction in the presence of an acid catalyst. In thereaction, together with the trioxane, there remains unreactedformaldehyde and water, which must be appropriately removed.

Trioxane is obtained, according to the conventional process, startingfrom aqueous solutions of formaldehyde the concentration of which canvary between 30 and 70%, when raising them to a boil in the presence ofan acid catalyst, which causes a trimerization reaction of theformaldehyde according to the following equation:3 CH₂O<==>(CH₂O)₃

The extent of the reaction is highly dependent on the formaldehydeconcentration in the solution and on the presence of trioxane therein.The greater the formaldehyde concentration and the smaller the trioxanecontent in the solution, the greater is the conversion.

The reaction takes place in a reactor and trioxane that is formed, in arelatively small amount due to the unfavorable balance, is vaporizedtogether with the remaining components of the mixture and must beseparated therefrom. The catalyst remains in the reactor, whichreceives, apart from the starting solution of formaldehyde, the residualcurrents of aqueous solutions of formaldehyde once this has beenseparated from the trioxane. This recirculating of residual currents ofaqueous formaldehyde solutions lowers the yield of the reaction, becausethey dilute the formaldehyde solution contained in the reactor and theyfurthermore lower the conversion rate, since they are accompanied by acertain amount of trioxane that sets back the formation reactionthereof.

The vapors originated in the reactor pass to a distillation column atthe head of which a mixture of trioxane, formaldehyde and water isobtained (that will hereinafter also be called an aqueous mixture oftrioxane and formaldehyde, both if it is in the liquid phase and if itis in the vapor phase) in an approximate proportion of 30–40% trioxane,17–30% formaldehyde and 40–50% water, which proportions depend on theconditions under which the reactor is operating. The excess formaldehydein the form of an aqueous solution is extracted from the foot of thecolumn and is recovered. The column distillate (i.e., the aqueousmixture of trioxane and formaldehyde) is extracted with awater-insoluble organic solvent, methylene chloride or benzene areexamples of the most used, that dissolves the trioxane, whereby theformaldehyde is left in the remaining aqueous phase. The trioxane has tobe recovered from these solutions in organic solvents by distillationand subsequent purification and the aqueous solution has to be treatedto recover the formaldehyde.

Owing to the multiple possibilities available for carrying out thediverse steps a large number of patents have been published, each aimedat improving one or another aspect of the process. Thus, there are thosethat use strong inorganic acids, such as sulphuric acid or phosphoricacid, as catalyst of the trimerization reaction and those which use ionexchange resins in acid form, heteropolyacids, zeolites,montmorillonites and other solid catalysts that have silica as base, tomention only a few.

Whatever the catalyst used, the vapors that originate in the reactor areformed by a mixture of trioxane, formaldehyde and water mainly and smallamounts of by-products such as methanol, formic acid, methyl formate,methylal, tetraoxane and dioxymethylene glycol dimethylether. Theobjective now is to separate the trioxane from the remaining products.In most patents this separation is achieved by means of a first step offractional distillation that allows the vapors at the head of the columnto be enriched in trioxane and a subsequent solvent extraction of thetrioxane from said mixture. Other separation processes are based on theintroduction of an inert gas into the mixture of vapors that facilitatesthe separation, azeotropic distillations with solvents or extractivedistillations with water or glycols (U.S. Pat. No. 3,281,336) thatretain the formaldehyde, or scrubbing of the gases at the exit from thereactor with heavy solvents that dissolve the trioxane (EP 0 680 959).There are further separation processes, such as distillation at twodifferent pressures or simultaneous reaction-extraction or with gases ina supercritical state. This multiplicity of processes shows how hard itis to find an optimum process.

It should be pointed out that in all cases, with the exception of theextractive distillation with water or glycols, the trioxane is extractedfrom the mixture with water and formaldehyde, obtaining a trioxanesolution in a solvent that must subsequently be removed in order toobtain pure trioxane. Only extractive distillation pursues the removalof the formaldehyde from the mixture and the release of the trioxanewhich separates out easily since it forms an azeotrope with the waterthat contains a high trioxane proportion. This is recovered bycrystallization from said azeotropic mixture. It is a process whichdispenses with the need for a solvent, whereby it eliminates theenvironmental problems that said solvents present. Nevertheless, it hasan additional problem, which is that the aqueous formaldehyde solutionobtained is highly dilute, whereby the formaldehyde has to beconcentrated for later recovery thereof. If the process is carried outwith a glycol or higher alcohol such as cyclohexanol, the formalobtained has to undergo a pyrolysis process to recover the formaldehyde.Both one case and the other require supplementary facilities and energythat complicate the process and make it more expensive.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome these drawbacks.This purpose is achieved by means of a process for the separation of anaqueous mixture of trioxane and formaldehyde, of the type firstmentioned above and obtained preferably from the vapors produced in thetrioxane formation reaction, including the step of reacting the aqueousmixture of trioxane and formaldehyde with urea and separating an exitingvapor phase and an exiting liquid phase, where the exiting vapor phasehas a second trioxane:formaldehyde ratio that is higher than the firsttrioxane:formaldehyde ratio.

It has been observed that the vapors produced by a solution formed bytrioxane, formaldehyde, urea and water contain more trioxane and lessformaldehyde than the starting solution. In fact, practically all theformaldehyde can be retained with the urea and, on the other hand, thetrioxane, together with water, passes to the vapor phase withoutappreciable amounts of formaldehyde, with the advantage of not having touse solvents or to add a formaldehyde separation and recovery step.

Thus, one way of separating the formaldehyde from the mixture oftrioxane, formaldehyde, water and other compounds formed in the trioxanesynthesis is by counterflow scrubbing of the vapors produced in thereactor with a concentrated solution of urea. The urea reacts with theformaldehyde to result in a urea-formaldehyde precondensate that lateron, in another facility, will produce the glues or resins used in thefiber board, plywood or laminate industry.

The reaction between formaldehyde and the urea is favored in an alkalinemedium, whereby it is advisable to add a certain amount of alkalitogether with the urea to the scrubbing solution, which in turn alsohelps to stabilize the resin solution obtained.

A further object of the present invention is the application of aprocess according to the invention for the separation of an aqueousmixture of trioxane and formaldehyde coming from a reactor in whichtrioxane is being synthesized, in particular when the trioxane issynthesized in the reactor starting from an aqueous solution offormaldehyde in the presence of an acid catalyst, and an aqueous mixtureof trioxane and formaldehyde in vapor phase is formed.

It is also possible to apply a process according to the invention whenthe aqueous mixture of trioxane and formaldehyde comes from adistillation column in which a prior aqueous mixture of trioxane andformaldehyde is separated from the excess formaldehyde, in particularwhen a prior aqueous mixture of trioxane and formaldehyde coming from areactor in which trioxane is being synthesized from an aqueous solutionof formaldehyde is distilled in the distillation column.

Finally it is possible to use a process of the invention for obtainingraw material for the production of urea-formaldehyde glues or resins.

A further advantage is that a less impure trioxane is already obtainedin this first step, because most of the impurities are retained in theurea solution, and where the low concentration of said impurities doesnot affect the quality of the resins that are subsequently obtained. Thetraces of trioxane that may remain have no influence either, since inthe course of the manufacture of the resins, it converts to formaldehydethat reacts with the urea present.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and characteristics of the invention will beappreciated from the following examples and figures wherein, without anylimitative character, some preferred embodiments of the invention arerelated. In the drawing:

FIG. 1 is a schematic view of a scrubbing facility or column accordingto the invention.

FIG. 2 is a schematic view of facility used for the realization of theexample.

DETAILED DESCRIPTION OF THE EMBODIMENTS

One embodiment of the invention consists of reacting a concentratedformaldehyde solution in an acid medium in a reactor according to one oranother of the existing processes and feeding the generated vapors to ascrubbing column. These vapors are supplied to a lower region of thecolumn but at a certain height above the foot to obtain an appropriateexhaustion of trioxane. The purpose of the scrubbing column is totransfer mass and energy between the incoming vapor mixture (that is theaqueous mixture of trioxane and formaldehyde) and an aqueous solution ofurea used as scrubbing liquid, at the same time as the formaldehyde andthe urea react together chemically. To achieve this mass transfer thecolumn is equipped with the appropriate devices normally used for suchpurpose, such as plates, or continuous fillers like those used in theconventional distillation columns. A concentrated urea solution and adilute sodium hydroxide solution (or, optionally, any other strong base)are supplied to the head of the column in order to maintain a pH above5, preferably between 7 and 9, in the solution that recirculates throughthe column. The formaldehyde and urea solution is collected at the footof the column, mostly as a urea-formaldehyde precondensate. A part ofsaid solution collected at the foot of the column, containing an amountof formaldehyde corresponding practically to the amount of formaldehydethat is supplied per unit of time into the column with the vaporsproduced in the reactor (i.e., with the aqueous trioxane andformaldehyde mixture), is removed from the system, and the rest of saidsolution collected at the foot of the column is recirculated to theupper region of the scrubbing column at a certain distance below thepoint where the fresh urea solution is supplied.

In this process there are four important parameters to keep in mind.These parameters are the amount of scrubbing liquid, the temperature ofsaid liquid, the pH thereof and the urea concentration therein.

The amount of scrubbing liquid, as well as the concentration of urea,must be chosen so that the solution obtained at the foot of thescrubbing column maintains a formaldehyde to urea molar ratio of between1:1 and 6:1, preferably between 4.5 and 5.5:1, so that the solutionobtained at the foot is appropriate for the later synthesis of thedesired glues or resins. Nevertheless, a larger proportion of urea maybe used, i.e. an F/U ratio of less than 1:1, which leads to an evengreater reduction of the amount of formaldehyde in the distillates, butas a result, a solution is obtained at the foot that will require anadditional treatment for its application to the preparation of glues orresins.

The temperature of the scrubbing liquid solution should be in the rangeof 92–100° C., preferably in the range of 94° to 98° C. At thisoperating temperature the formaldehyde vapor pressure of the scrubbingliquid solution is relatively high, whereby it is necessary for thefresh urea solution to be supplied at the upper region of the scrubbingcolumn in order to retain all the formaldehyde separated out of saidliquid scrubbing solution.

In a variant of the invention and with the purpose of maintaining thetemperature of the scrubbing liquid solution below the above-mentionedvalues so as thereby to reduce the proportion of formaldehyde given off,a pressure below atmospheric pressure is used. In this way thetemperature can be controlled and the separation of the trioxane fromthe formaldehyde can be promoted.

In another variant of the invention, if it is wanted to limit theproduction of urea-formaldehyde precondensates, thetrioxane-formaldehyde-water mixture (or trioxane and formaldehydeaqueous mixture) to be scrubbed in a separate scrubbing column with theurea solution, is the one that is distilled at the head of thedistillation column used in a conventional production process like theone described at the beginning of the text. In this case, the distillatefrom said distillation column contains a lower formaldehydeconcentration and on the other hand this concentrates at the foot of thedistillation column. If the concentration of the solution at the foot ofsaid distillation column is sufficiently high, it may be reccirculatedto the reactor, if it were not, it must be concentrated in an adjoiningfacility. In this variant, the formaldehyde solution returned to thereactor is accompanied by a certain amount of trioxane that, as alreadysaid at the beginning, reduces the conversion rate in the reactor.

Both in one case and in the other, the trioxane-water azeotrope isobtained at the head of the urea scrubbing column in the form of vapourthat condenses and the trioxane is subsequently crystallized from saidcondensate by cooling. The small amounts of impurities that accompany itare easily removed together with the mother liquors. The separation andlater purification process can follow any of the ways already describedin the literature and does not form a part of the present invention.

Vapors coming from a trioxane synthesis reactor, or vapors exiting ahead of a distillation column of a conventional process, are supplied tothe scrubbing column 1 at point 2. They form an aqueous trioxane andformaldehyde mixture. Said vapors (or aqueous trioxane and formaldehydemixture) are scrubbed in the scrubbing column 1 with the recirculatingscrubbing liquid. This scrubbing liquid is extracted from the foot 3 ofthe scrubbing column 1 and is reinserted in part in the scrubbing column1 by a pump 4 at point 6. A urea and sodium hydroxide solution issupplied into the scrubbing column 1 at an upper region 7 thereof and atrioxane-water azeotrope is obtained through the head 8 of saidscrubbing column 1. From the lower exit 5 there is removed from thecurrent of scrubbing liquid extracted at the foot 3 of the scrubbingcolumn 1 an amount of formaldehyde corresponding to the formaldehydesupplied at point 2 and it is taken to a facility for the production ofurea-formaldehyde resins.

In the following examples the effectiveness of the treatment with ureais demonstrated. The facility used to carry out these experiments is asshown in FIG. 2. The mixture of trioxane, formaldehyde, water and ureais introduced in the reactor having a double thermostatted jacket 21provided with a reactant inlet tube 22 and a vapor outlet tube 23 aswell as with a sampling point 24. The pH of the mixture is adjusted witha sodium hydroxide solution and the mass is heated to boiling point. Thevapors produced are absorbed in water contained in appropriate waterscrubbers 25. The content of the reactor 21 and the liquors of the waterscrubbers 25 are sampled at set intervals of time and the formaldehydeand trioxane content thereof are determined.

In the following examples, reflected in Table 1, the influence of theamount of added urea as well as the effect of the pH on the formaldehydeemission in the vapors is seen. As a measure of the effectiveness of theprocess the % Trioxane/% Formaldehyde (T/F) ratio and the “enrichment”R, defined as the ratio of the T/F ratios in the distillate and in thereactor, respectively, is given.

Example number 1, without urea, is taken as a reference.

TABLE 1 Operation 1 2 3 4 5 6 7 Reactor Formaldehyde % 20.6 19.1 16.216.9 18.4 19.8 26.7 Trioxane % 30.0 27.3 23.1 23.1 26.3 28.1 2.6 Water %49.4 44.5 37.7 37.1 43 45.9 62.3 Urea % 0 9.1 23 23 12.3 6.2 8.4 pH 5.45.5 5.5 7.5 5.4 7.2 7.5 F/U mol/mol 4.2 1.4 1.47 3.0 6.4 6.35 % TOX/% F1.46 1.43 1.43 1.37 1.43 1.42 0.098 Distillate Formaldehyde % 11.6 6.42.5 1.2 5.4 6.4 17.8 Trioxane % 64.7 60.5 62.6 67.3 58.8 60.2 7.9 Water% 23.7 33.1 34.9 31.5 35.8 33.4 74.3 % TOX/% Form. 5.6 9.4 24.7 56.1 6.49.4 0.44 Enrichment R 3.8 6.6 17.3 40.9 4.5 6.6 4.5

1. A process for the separation of an aqueous mixture of trioxane andformaldehyde, said mixture having a first trioxane to formaldehyderatio, said process comprising the steps of reacting the aqueous mixtureof trioxane and formaldehyde, which is in vapor phase, with a ureasolution; wherein said reacting step includes the steps of scrubbingsaid aqueous mixture of trioxane and formaldehyde with the urea solutionin a scrubbing column; and separating an exiting vapor phase and anexiting liquid phase, wherein said exiting vapor phase has a secondtrioxane to formaldehyde ratio that is higher than said first trioxaneto formaldehyde ratio and said exiting liquid phase is aurea-formaldehyde solution, and wherein, at a foot of said scrubbingcolumn, a formaldehyde to urea molar ratio is between 1:1 and 6:1. 2.The process of claim 1, further comprising the step of adding a dilutesolution of a strong base at a head of said column together with saidurea solution.
 3. The process of claim 1, wherein said formaldehyde tourea molar ratio is between 4:5:1 and 5:5:1.
 4. The process of claim 1,wherein said urea solution at said foot of said column has a temperatureranging from 92° to 100° C.
 5. The process of claim 4, wherein said ureasolution at said foot of said column has a temperature ranging from 94°to 98° C.
 6. The process of claim 1, wherein said scrubbing columnoperates at an internal pressure equal to or lower than atmosphericpressure.
 7. The process of claim 1, wherein said urea solution at saidfoot has a pH above
 5. 8. The process of claim 7, wherein said ureasolution at said foot has a pH ranging from 7 to
 9. 9. The processaccording to claim 1, further comprising the step of obtaining thetrioxane from a reactor in which trioxane is being synthesized.
 10. Theprocess of claim 9, further comprising the step of synthesizing thetrioxane in said reactor starting from an aqueous solution offormaldehyde in the presence of an acid catalyst.
 11. The processaccording to claim 1, further comprising the step of deriving saidaqueous mixture of trioxane and formaldehyde from a distillation columnin which a prior aqueous mixture of trioxane and formaldehyde isseparated from excess formaldehyde.
 12. The process according to claim11, further comprising the step of distilling said prior aqueous mixtureof trioxane and formaldehyde coming from a reactor in which trioxane isbeing synthesized from an aqueous solution of formaldehyde.
 13. Theprocess according to any one of claims 1, 2, 3, 4, 5, 6, 7 and 8,further comprising the step of using the urea-formaldehyde solution inthe production of urea-formaldehyde glues or resins.